1. Field of the Invention
The present invention relates to an injection control method and system for internal combustion engines, in particular Diesel engines.
The invention specifically relates to an injection control method for controlling a CR (Common Rail) fuel injection system in a Diesel engine.
The invention relates, particularly but not exclusively, to an injection control method and system for a direct injection Diesel engine, and the following description will make reference to this application for convenience of illustration only.
2. Description of the Related Art
As it is well known, the strict restrictions on the emissions and fuel usage in internal combustion engines enacted by the European Union for application by the year 2005, along with the latest technological developments of fuel injection systems, have focused the attention of the automobile industry on the optimization of the engine fuel injection process.
At the same time, the market of Diesel-powered cars has increased from 20 to 35%, the expansion being ascribable to the superior thermodynamic efficiency of Diesel engines compared to gasoline engines, in the face of a potential for pollution that is still fairly high.
This favorable trend to the Diesel engine opens new prospects for research on fuel injection systems, fuel injection being the only input of this type of engines that can be manipulated.
Another requisite is observance of the limits enforced by standing European regulations (EURO IV and V), schematically shown in FIG. 1.
Accordingly, the Diesel engine industry is thoroughly investigating the operation of fuel injection systems, including injectors, in order to find out a flexible solution that can cope with statutory limitations.
The study of fuel injection systems must take account of that variations in the number of injections per working cycle substantially modify the percentages of polluting matter. Also the changes in percentage are of opposite signs for some substances: for example, carbon monoxide CO drastically decreases as the number of injections increases, whereas “white smoke” or hydrocarbons HC increases with the number of injections, as it is shown in FIG. 2.
FIG. 2 is a comparative graph of the percentages of combustion noise (A), specific consumption (B), emissions of nitrogen oxides NOx(C), hydrocarbons HC (E), carbon monoxide CO (F), and particulate (D) under the following conditions of operation:                one pilot injection (Pilot) and one main injection (Main);        one initial injection (Pre) and one main injection (Main);        one initial injection (Pre), one main injection (Main), and a later injection (After); and        one initial injection (Pre), a first main injection (Main1) at 50%, and a second main injection (Main2) at 50%.        
It should be noted that the pilot injection and pre-injection are pulse injections, whereas the main injections last longer.
It should be further considered that fuel injection systems are presently used to serve high performance engines as made available on a large scale by recent developments in the Diesel field. These high performance engines use less fuel and exhibit much reduced carbon monoxide CO2, gaseous and particulate emission values.
A comparison of the “old” indirect injection Diesel engine (IDI engine) with the “new” direct injection engine (DI engine) can help to illustrate the development.
The basic difference between an IDI engine, schematically shown at 1 in FIG. 3, and a DI engine is in the injection pressure of the injection system and in the manner of producing and burning the fuel/air mixture.
The engine 1, specifically its engine cylinder 2, includes a small swirl chamber 5 in the cylinder head 3 of the cylinder 2, which opens to the main combustion chamber placed in the head 7 of the piston 9 through a passage having suitable dimensions. The swirl chamber 5 is connected to an injector 4 and a glow plug 6. Also shown in FIG. 3 is a valve 8.
The function of the swirl chamber 5 is the one of optimizing the formation of the fuel/air mix and of the following combustion to be completed in the cylinder 2.
This combustion mode is at least 15% less efficient than that to be obtained by injecting the fuel directly into the cylinder as it is done in ID engines of recent manufacture.
IDI engines show, in fact, higher load and thermal losses through the mix transfer and combustion areas compared to ID engines.
The combustion process in a Diesel engine is typically heterogeneous, in the sense that fuel and air are not mixed together before combustion but are only contacted after the air temperature has been raised (about 500° to 600° C.) by compression in the cylinder to ignite the mix.
Until recently, the direct injection of fuel, which constitutes a significant step forward, was impracticable especially in “light-duty” engines which are conceived for higher rotational speeds than standard truck engines, because of engineering and operating problems, such as noise emission and rugged power output.
To improve on these limitations and make direct fuel injection a practical proposition, pumps and electronic control arrangements have been developed that afford higher injection pressures.
In particular a so-called “common rail” (CR) injection control system, schematically shown in FIG. 4, has been recently introduced. The CR system allows to reach enough high injection pressures such to obtain the fuel spraying in the combustion chamber, that results in a near-perfect fuel/air mix.
A CR injection system basically comprises a high-pressure radial-piston feed pump, a rail, a set of injectors connected to the high-pressure pipe, a control unit, actuators, and a plurality of sensors. The pump maintains the fuel under a high pressure and delivers it into the rail that serves all the injectors and essentially acts as a reservoir. Part of the fuel is then injected into the combustion chamber by the injectors receiving an electromagnetic command, and the rest of the fuel is returned to the fuel tank to be recycled.
The circulating fuel flow is established and monitored by sensors connected to an electronic control unit, where the pressure recorded by the sensors is compared with predetermined values and thus overpressure is driven by returning the exceeding fuel to the tank. The information from the sensors enables the control unit to adjust the amount of fuel to be injected according to the engine load and RPM, thereby providing for highly flexible management.
The pressure so produced meets the engine requirements at all ranges, unlike traditional systems where the pump was linked to the engine operation such that the pressure depended on the engine RPM and was never at optimum levels, especially at low speeds.
Further, in fast diesel engines, as light duty diesels are, mixing time must be the shortest as possible in order to ensure the engine desired performances. The innovative aspect of the CR system is that high pressures (up to 1600 bar) can be produced independently of the engine speed, so that the right amount of fuel is delivered for optimum fuel/air mixing and combustion under all conditions.
In engines already in the field and those still in the laboratory stage, the CR system is controlled by pre-set mapping. In practice, a pressure sensor mounted in the rail senses a voltage signal between 0 and 5 Volts and sends it to the control unit, where the engine operation maps (or matrices) are implemented.
Particularly in the operation maps, a duty cycle of the pressure regulator placed in the high-pressure pump corresponds to each voltage value.
Thus, when the rail pressure sensor senses the required pressure to meet a load variation and the control unit maps the appropriate duty cycle to produce that pressure (the value of the duty cycle being a function of the engine RPM), the system settles, ensuring that the injection occurs correctly.
A limitation to conventional injection control systems comes inherently from their operating method, wherein the duty cycle of the pressure regulator is set according to fixed maps. It is evident that such maps cannot account for transients and mechanical variations due to an ageing engine.
Also, the electronic control unit will decide on the duration of the injection (and, therefore, the fuel flow rate) according to the load demand once the optimum pressure is established, and with it the torques and power outputs of the engine for that speed.
It is recalled that currently available CR systems only effect two injections (the Pilot and Main injections) per cycle. However, more recent studies have led to new generation systems effecting five injections per cycle (called the Pilot, Pre, Main, After and Post injections). FIGS. 5A and 5B schematically show the injections for traditional (FIG. 5A) and new generation (FIG. 5B) systems.